Blood is the life-maintaining fluid that circulates through the body's heart, arteries, veins, and capillaries. It carries away waste products and brings nourishment, electrolytes, hormones, vitamins, antibodies, heat, and oxygen to the tissues.

Because the functions of blood are many and complex, many disorders require clinical care by a physician or other health care professional. These conditions include anemias and bleeding disorders, as well as cancers of the blood.

Hematology is the scientific study of blood and blood-forming tissues. A hematologist is a physician who specializes in the functions and disorders of the blood.